1. Field of the Invention
The present invention relates to a multilevel data transmission system employing the so-called class IV partial response shaping.
2. Description of the Prior Art
As an introduction to the description of the present invention, an outline of a coder and a decoder based on the partial response shaping of the (1, 0, -1) format will be explained with reference to the drawings. FIG. 1(a) is a diagram of a fundamental coder structure based on the partial response shaping of the (1, 0, -1) format (hereinafter referred to simply as "partial response shaping"), and FIG. 1(b) is a diagram of a fundamental decoder structure for use with the same shaping technique. In these figures is illustrated a case of 2N--1 kinds of levels (N being an integer not smaller than 3). More particularly, in the coder of the transmitter side, input signals consisting of N kinds of symbols S.sub.0, S.sub.1, . . . S.sub.N-1 are subjected to modulo-N addition with its output signals as delayed by 2T (T being a clock time period for data transmission). The resultant output signals are further modulo-N substraction with said output signals as delayed by 2T. Accordingly, the output signals of the substractor may possibly have 2N--1 kinds of levels -(N--1), -(N--2), . . . -1, 0, 1, . . ., (N--1). In the receiver-side decoder shown in FIG. 1(b), these signals are subjected to modulo-N addition with a number N. In this data transmission system, since a symvol Si (i = 1, 2, . . . N--1) of the input signals on the transmitter side corresponds to either level i or level -(N--i) in the transmitter output, the decoding can be performed easily, and upon occurrence of an error in code during transmission the error would not propogate on the receiver side.
With regard to the above-outlined partial response shaping of the (1, 0, -1) format, reference should be made to the following publications:
1. "A New Signal Format For Efficient Transmission" by F. K. Becker, E. R. Kretzmer and J. R. Sheehan, B.S.T.J. Vol. XLV, NO. 5, pp. 755-758;
2. "Generalization of a Technique for Binary Data Communication" by E. R. Kretzmer, IEEE Trans. Comm. Tech., 1966, pp. 67-68; and
3. "Multilevel Partial-Response Signalling" by A. M. Gerrish and R. D. Howson, ICC '67 Record 19CP67-1181, p. 186.
In such a partial response shaping, (1) in the case of transmitting a multilevel signal in the form of a base band signal, it should be taken into consideration whether or not the signal polarity is inverted in an amplifier and the like, while (2) in the case of transmitting a multilevel signal in the form of a band-limited amplitude-modulation signal, the polarity of the base band signal after demodulation may be possibly inverted owing to the fact that the phase of a demodulated carrier wave for use in synchronous detection is shifted by 180.degree.. With respect to the case (2) above, it is a common practice in the VSB or SSB receiver to provide a phase control circuit for demodulated carrier waves such that a quadrature component in the demodulated base band signal may be maintained at zero. However, it is well-known that in such a case the phase of the demodulated carrier wave may be possibly shifted by 180.degree.. If the polarity of the multilevel signal is inverted in this way, the partial response shaping results in level i of the transmitter output being received as level (-i) whose demodulated symbol is S.sub.N-i, and thus level i and level N--i are no longer identical to each other except for level O if N is an odd number, or except for symbols 0 and N/2 if N is an even number. Similarly, in the case where level -(N--i) has been transmitted after partial response shaping in response to a transmitter input symbol Si, lelvel N--i would also be received owing to polarity inversion, resulting in demodulation of symbol S.sub.N-i, and thus the data cannot be transmitted correctly. In this way, upon occurrence of polarity inversion, symbol Si would be always mistaken for symbol S.sub.N-i. However, the above-referred symbbols S.sub.O and S.sub.N/2 are transmitted correctly even upon occurence of polarity inversion. In other words, it can be seen that symbols S.sub.i and S.sub.N-i form a symbol pair which would be incorrectly transmitted upon occurrence of polarity inversion, where i is a positive integer smaller than N/2.